Fine carbon fiber, method for producing the same and electrically conducting material comprising the fine carbon fiber

ABSTRACT

A fine carbon fiber having an outer diameter of about 1 to about 80 nm and an aspect ratio of 10 to 30,000, comprising a hollow center portion and a multi-layer sheath structure of a plurality of carbon layers, the layers forming annual rings, wherein the sheath-forming carbon layers form an incomplete sheath, i.e., the carbon layers are partially broken or disrupted in a longitudinal direction, and the outer diameter of the carbon fiber and/or the diameter of the hollow center portion are not uniform in a longitudinal direction. The carbon fiber is obtained by instantaneously reacting a carrier gas at a high temperature and an organic compound gas kept at a temperature below the decomposition temperature of the transition metal compound and has a conductivity equivalent to that of a conventional vapor phase method and is useful as a filler material in resins, rubbers, paints and the like.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 10/254,674, filed on Sep.26, 2002 is now U.S. Pat. No. 6,699,582, which is a divisional ofapplication Ser. No. 09/832,792 filed Apr. 12, 2001, now U.S. Pat. No.6,489,025, which claims benefit of Provisional Application No.60/268,058, filed Feb. 13, 2001, the disclosures of which areincorporated herein by reference. This application is based on JapanesePatent Application No. 2001-10675 filed Apr. 12, 2000, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a fine carbon fiber having a specificstructure and to a production process therefor; and more particularly toa fine carbon fiber suitable as a filler used in composite materials,such as resin- or rubber-based composite materials, and to a productionprocess for the carbon fiber. The present invention also relates to aconducting material comprising such a fine carbon fiber.

BACKGROUND OF THE INVENTION

Carbon fibers are used in a variety of composite materials because oftheir excellent characteristics such as high strength, high elasticmodulus, and high conductivity. In addition, carbon fibers exhibitexcellent mechanical strengths. Due to their conductivity, carbon fibersor carbon materials can be utilized in a variety of fields. In recentyears, in conjunction with developments in electronic techniques, carbonfibers have been regarded as a promising filler in conducting resins forproducing electromagnetic shielding materials or antistatic materials.Also, with the trend that resins have come to be used in the manufactureof automobiles in order to reduce their weight, carbon fibers have beenseen as a useful antistatic filler that can be incorporated into theresins employed in automobiles.

Conventional carbon fibers, i.e., organic carbon fibers, are produced ona large scale by subjecting organic fibers, such as PAN-, pitch-, orcellulose-based fibers, to heat treatment and carbonization. In general,when carbon fibers are used as a filler in fiber-reinforced compositematerials, in order to increase the contact area between the carbonfiber and the matrix of the material, the diameter of the fiber isreduced or the length thereof is increased. As a result, thereinforcement effect on the composite material is enhanced. In order toimprove adhesion between the carbon fiber and the matrix, the carbonfiber preferably has a rough surface rather than a smooth surface.Therefore, the carbon fiber is subjected to surface treatment. Forexample, the carbon fiber is oxidized by exposure to air at a hightemperature, or a coating agent is applied onto the surface of thefiber.

However, conventionally, it has been impossible to produce fine carbonfibers since the filament of organic fiber as the raw material has adiameter of at least about 5 to 10 μm. Furthermore, the ratio of lengthto diameter (i.e., aspect ratio=length/diameter) of conventional carbonfiber is limited. Because of these limitations, there has been a keendemand for the development of carbon fibers of a small diameter and ahigh aspect ratio.

When carbon fibers are incorporated into resins used for producing anautomobile body, or in resins or rubbers for producing an electronicdevice, the carbon fibers must have conductivity comparable to that of ametal. Therefore, carbon fibers serving as a filler material have beenrequired to have an improved conductivity.

In order to improve conductivity, carbon fibers must be subjected tographitization to thereby increase the degree of crystallinity.Therefore, carbon fibers are usually subjected to graphitization at ahigh temperature. However, even when carbon fibers are subjected to sucha graphitization, they still fail to attain conductivity comparable tothat of a metal. Therefore, when a composite material is produced by useof a carbon fiber, in order to compensate for a low conductivity ofcarbon fiber itself, a large amount of carbon fiber is to beincorporated into the composite material. However, in this case, theworkability and mechanical characteristics of the composite material areimpaired. Therefore, in view of practical use, it is necessary to makefurther improvements to the conductivity of carbon fiber. In addition,it is also necessary to enhance the strength of the carbon fiber byreducing its diameter.

In the late 1980's, a vapor grown carbon fiber was produced through aprocess that differed from that used for producing a carbon fiberthrough carbonization and graphitization from an organic fiber such asPAN.

The vapor grown carbon fiber (hereinafter abbreviated as “VGCF”) isproduced through thermal decomposition of hydrocarbon gas in a vaporphase in the presence of a metallic catalyst. Through this process, acarbon fiber having a diameter of hundreds of nm to 1 μm can beproduced.

A variety of processes for producing VGCF are known, including a processin which an organic compound such as benzene, serving as a raw material,and an organic transition metal compound such as ferrocene, serving as ametallic catalyst, are introduced into a high-temperature reactionfurnace together with a carrier gas, to thereby produce VGCF on asubstrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700);a process in which VGCF is produced in a free state (Japanese PatentApplication Laid-Open (kokai) No. 60-54998); and a process in which VGCFis grown on a reaction furnace wall (Japanese Patent ApplicationLaid-Open (kokai) No. 7-150419).

Through the aforementioned processes, there can be produced a carbonfiber of a relatively small diameter and a high aspect ratio thatexhibits excellent conductivity and is suitable as a filler material.Therefore, a carbon fiber having a diameter of about 100 to 200 nm andan aspect ratio of about 10 to 500 is mass-produced, and is used, forexample, as a conducting filler material in conducting resins or as anadditive in lead storage batteries.

A characteristic feature of a VGCF filament resides in its shape andcrystal structure. A VGCF filament has a structure including a very thinhollow part in its center portion, and a plurality of carbon hexagonalnetwork layers whose crystals surround the hollow part like annualrings.

However, conventionally, VGCF having a small diameter of less than 100nm cannot be produced on a large scale.

Recently, Iijima, et al. have discovered a multi-layer carbon nano-tube,which is a type of carbon fiber having a diameter smaller than that ofVGCF, derived from soot obtained by evaporating a carbon electrodethrough arc discharge in helium gas. The multi-layer carbon nano-tubehas a diameter of 1 to 30 nm, and is a fine carbon fiber filament havinga structure similar to that of a VGCF filament. That is, the tube has astructure including a hollow part in its center portion, and a pluralityof carbon hexagonal network layers whose crystals are superimposed inthe form of annular rings around the hollow part.

However, the above process for producing the nano-tube through arcdischarge is not carried out in practice, since the process is notsuitable for mass production.

Meanwhile, a carbon fiber having a high aspect ratio and exhibiting ahigh conductivity could possibly be produced through the vapor-growthprocess, and therefore attempts have been made to improve thevapor-growth process for the production of carbon fiber of a smallerdiameter. For example, U.S. Pat. No. 4,663,230 and Japanese PatentExamined Publication (kokoku) No. 3-64606 disclose a graphiticcylindrical carbon fibril having a diameter of about 3.5 to 70 nm and anaspect ratio of 100 or more. The carbon fibril has a structure in whicha plurality of layers of ordered carbon atoms are continuously disposedconcentrically about the cylindrical axis of the fibril, and the C-axisof each of the layers is substantially perpendicular to the cylindricalaxis. The entirety of the fibril includes no thermally decomposed carbonovercoat deposited through thermal decomposition, and has a smoothsurface.

Japanese Patent Application Laid-Open (kokai) No. 61-70014 discloses acarbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2to 30,000, which fiber is produced through a vapor-growth process.According to this publication, a carbon layer obtained through thermaldecomposition has a thickness of 20% or less of the diameter of thecarbon fiber.

The both carbon fibers described above have smooth fiber surfaces. Theyexert substantially no frictional force because their surfaces are lessuneven and they exhibit poor chemical reactivity since they have smoothfiber surfaces. Therefore, when such a carbon fiber is used in acomposite material, the fiber must be subjected to surface treatment;for example, the surface of the fiber must be subjected to asatisfactory degree of oxidation.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a fine carbon fiber, which has a diameter of less than 100 nm,exhibits a high conductivity and an excellent adhesion to materials suchas a resin or rubber, and is suitable as a filler material.

Another object of the present invention is to provide a method forproducing such a carbon fiber.

Still another object of the present invention is to provide a conductingmaterial comprising such a carbon fiber.

The present inventors provide a novel carbon fiber having a structuretotally different from that of a conventional carbon fiber. Accordingly,the present invention provides:

-   1) A fine carbon fiber having an outer diameter of about 1 to about    80 nm and an aspect ratio of 10 to 30,000, comprising a hollow    center portion and a multi-layer sheath structure of a plurality of    carbon layers (also referred to as “carbon sheets”), the layers    forming concentric rings, wherein the sheath-forming carbon layers    form an incomplete sheath, i.e., the carbon layers are partially    broken or disrupted in a longitudinal direction, and the outer    diameter of the filament and/or the diameter of the hollow center    portion are not uniform in a longitudinal direction;-   2) The fine carbon fiber according to 1) above, wherein the    thickness or structure of the carbon layers is partially    asymmetrical with respect to the hollow center portion;-   3) The fine carbon fiber having an outer diameter of about 1 to    about 80 nm and an aspect ratio of 10 to 30,000, comprising the fine    carbon fiber as recited in 1) or 2) above in an amount of about 10    mass % or more;-   4) The fine carbon fiber obtained through heat treatment of the fine    carbon fiber as recited in any one of 1) to 3) above;-   5) The fine carbon fiber according to 4) above, wherein the heat    treatment is carried out at about 900 to about 3,000° C.;-   6) A fine carbon fiber having an outer diameter of about 1 to about    80 nm, an aspect ratio of 10 to 30,000, R value (I_(D)/I_(G)) by    Raman spectrophotometry of about 0.6 to about 1.6, and an interplane    distance C₀ by X-ray diffraction of 6.70 to 6.95 Angstroms, and    having a cross-section perpendicular to the longitudinal direction    of fiber being of a polygonal shape, the fiber comprising a hollow    center portion and a multi-layer sheath structure of a plurality of    carbon layers;-   7) A fine carbon fiber having an outer diameter of about 1 to about    80 nm, an aspect ratio of 10 to 30,000, R value (I_(D)/I_(G)) by    Raman spectrophotometry of about 0.1 to about 1, and an interplane    distance C₀ by X-ray diffraction of 6.70 to 6.90 Angstroms, and    having a cross-section perpendicular to the longitudinal direction    of filament being of a polygonal shape, the fiber comprising a    hollow center portion and a multi-layer sheath structure of a    plurality of carbon layers;-   8) The fine carbon fiber according to 6) or 7) above, having an    outer diameter of about 1 to about 80 nm and an aspect ratio of 10    to 30,000, and having a cross-section perpendicular to the    longitudinal direction of filament being of a polygonal shape, each    filament comprising a hollow center portion and a multi-layer sheath    structure of a plurality of carbon layers in the form of annular    rings around the hollow part;-   9) The fine carbon fiber having an outer diameter of about 1 to    about 80 nm and an aspect ratio of 10 to 30,000, comprising the fine    carbon fiber as recited in any one of 6) to 8) above in an amount of    about 10 mass % or more;-   10) A method for producing fine carbon fiber, which process    comprises a step of causing an organic compound solution containing    an organic transition metal compound and, optionally a sulfur    compound to vaporize, and feeding the vaporized solution to a    reaction furnace while the temperature of the solution is maintained    below the decomposition temperature of the organic transition metal    compound; a step of feeding a carrier gas which has been heated to a    high temperature to the reaction furnace through a path separate    from that of the solution; and a step of causing the vaporized    solution and the carrier gas to be combined in a heated reaction    zone of about 700 to about 1,300° C. in the reaction furnace, to    thereby carry out reaction instantaneously;-   11) The method for producing fine carbon fiber according to 10)    above, wherein the preliminary heating temperature is about 500 to    about 1,300° C.;-   12) The fine carbon fiber according to any one of 1) to 3) and 6) to    9), produced by a production process comprising a step of causing an    organic compound solution containing an organic transition metal    compound and, optionally a sulfur compound to vaporize, and feeding    the vaporized solution to a reaction furnace while the temperature    of the solution is maintained below the decomposition temperature of    the organic transition metal compound; a step of feeding a carrier    gas which has been heated to a high temperature to the reaction    furnace through a path separate from that of the solution; and a    step of causing the vaporized solution and the carrier gas to be    combined in a heated reaction zone of about 700 to about 1,300° C.    in the reaction furnace, to thereby carry out reaction    instantaneously;-   13) The fine carbon fiber according to any one of 1) to 3) and 6) to    9), comprising further subjecting to heat treatment the fine carbon    fiber produced by a production process comprising a step of causing    an organic compound solution containing an organic transition metal    compound and, optionally a sulfur compound to vaporize, and feeding    the vaporized solution to a reaction furnace while the temperature    of the solution is maintained below the decomposition temperature of    the organic transition metal compound; a step of feeding a carrier    gas which has been heated to a high temperature to the reaction    furnace through a path separate from that of the solution; and a    step of causing the vaporized solution and the carrier gas to be    combined in a heated reaction zone of about 700 to about 1,300° C.    in the reaction furnace, to thereby carry out reaction    instantaneously;-   14) The fine carbon fiber according to 13) above, wherein the heat    treatment temperature is about 900 to about 3,000° C.;-   15) The fine carbon fiber according to any one of 12) to 14) above,    wherein the preliminary heating temperature is about 500 to about    1,300° C.;-   16) A conducting material comprising the fine carbon fiber according    to any one of 1) to 9) and 12) to 15).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation showing an example of thestructure of the fine carbon fiber of the present invention.

FIG. 1B is a schematic representation showing another example of thestructure of the fine carbon fiber of the present invention.

FIG. 2 shows a schematic representation of a production apparatus usedin the Example, which includes a vertical heating furnace.

FIG. 3 illustrates a carbon fiber produced through a conventionalvapor-growth process.

FIG. 4 illustrates a fine carbon fiber produced through the process ofthe present invention.

FIG. 5A is a transmission electron micrograph of the fine carbon fiberof the present invention.

FIG. 5B is a transmission electron micrograph of the fine carbon fiberof the present invention.

DESCRIPTION OF THE PRESENT INVENTION

Hereinafter, the present invention will be described in detail.

A characteristic feature of the production process for fine carbon fiberof the present invention resides in that an organic compound solution,serving as raw material and also a catalyst, is vaporized and fed to areaction furnace while the temperature of the solution is maintainedbelow the decomposition temperature of an organic transition metalcompound serving as a catalyst; a carrier gas which has been heated to ahigh temperature is fed to the reaction furnace through a path separatefrom that of the vaporized solution; and the vaporized solution and thecarrier gas are combined instantaneously in a heated reaction zone of700 to 1,300° C. in the reaction furnace for the first time. Since thecarrier gas can be fed to the reaction zone through a path separate fromthat of the vaporized raw material in a state where it is heated to atleast the decomposition temperature of the organic transition metalcompound, i.e., a temperature in the vicinity of the reactiontemperature, reaction proceeds rapidly after the raw material and thecarrier gas are combined. Since the carrier gas is fed through a pathseparate from that of the vaporized raw material, the raw material isnot heated to an abnormally high temperature. Therefore, decompositionof the transition metal compound is not initiated before it can get inthe reaction furnace. As a result, a fine carbon fiber is grown in thefurnace.

In the production process for a fine carbon fiber of the presentinvention, the organic transition metal compound that can be used as ametallic catalyst is a compound including at least one species selectedfrom metal elements belonging to the groups IVa, Va, VIa, VIIa, and VIIIin the periodic table. Preferably, a metallocene compound such asferrocene or nickelocene is used. In the present invention, the amountof the transition metal in the catalyst is about 0.03 to about 10.0 mass%, preferably about 0.1 to about 5.0 mass %, on the basis of the amountof carbon in the catalyst.

In addition, a sulfur compound may be used as a promoter. The type ofthe sulfur compound is not particularly limited, so long as it dissolvesin an organic compound such as benzene or toluene serving as a carbonsource. Examples of such sulfur compounds include thiophene, thiols, andinorganic sulfur. The amount of the sulfur compound is about 0.01 toabout 5.0 mass %, preferably about 0.1 to about 3.0 mass %, on the basisof the organic compound.

Examples of organic compounds serving as a carbon source for producing acarbon fiber include benzene, toluene, xylene, methanol, ethanol,naphthalene, phenanthrene, cyclopropane, cyclopentene, cyclohexane, andmixtures thereof. Alternatively, volatile oil, kerosene, or gas such asCO, natural gas, methane, ethane, ethylene, or acetylene may be used asa carbon source. Of these, an aromatic compound, such as benzene,toluene, or xylene, is particularly preferred.

Hydrogen gas is usually used as a carrier gas. In the present invention,the carrier gas is heated in advance, preferably to 500 to 1,300° C.,more preferably 700 to 1,300° C. The carrier gas is heated so that uponthe reaction, the formation of metallic grains of catalyst can occursimultaneously with the supply of a carbon source due to the thermaldecomposition of an organic compound, to thereby allow the reaction tooccur instantaneously.

In the case in which the carrier gas is combined with a raw material gascontaining an organic compound and a transition metal compound, thetemperature of the carrier gas lower than 500° C. does not allow thethermal decomposition of the organic compound serving as a raw materialto occur easily. In contrast, when the temperature of the carrier gas ishigher than 1,300° C., carbon fiber is grown in its radial direction,and the diameter of the fiber tends to become large.

In the production process of the present invention, appropriateproportions by mol % of a transition metal compound, an organiccompound, and carrier gas are about (0.005–0.2):(0.5–6):(94–99.5), andtheir total amount is 100 mol %.

In order to produce carbon fiber having a minute diameter, wherehydrogen gas is used as the carrier gas, the proportion of hydrogen gasis set at 90 mol % or more, preferably 94 mol % or more, and morepreferably 96 mol % or more; i.e., the proportion (mol %) of the organiccompound serving as a carbon source is preferably lowered.

When ferrocene is used as a transition metal compound, the vaporized rawmaterial gas must be maintained at a temperature within a range of 200to 400° C. until the gas is fed to a reaction furnace. When thetemperature of the vaporized raw material gas is in excess of 450° C.,the vaporized organic transition metal compound thermally decomposes,and the atomized transition metal begins to aggregate. Unlessdecomposition of the organic compound serving as the carbon sourceaccompanies the decomposition of the transition metal compound, nocarbon fiber is produced. A carbon fiber is grown on the atomizedtransition metal serving as a nucleus, and thus the size of the metaldetermines the diameter of the grown carbon fiber. Briefly, when thetransition metal particles aggregate and the secondary particle size ofthe resultant metal aggregation becomes large, the diameter of thecarbon fiber grown on the aggregation becomes large. Therefore, thetemperature of the raw material gas must be maintained below thedecomposition temperature of the transition metal compound until the gasis fed to the reaction furnace.

Preferably, the raw material gas containing an organic compound and atransition metal compound is instantaneously introduced into a heatedzone in the reaction furnace while the temperature of the raw materialgas is maintained below the decomposition temperature of the transitionmetal compound, and simultaneously a carrier gas (e.g., hydrogen gas)heated at 500 to 1,300° C. is introduced into the zone through a pathseparate from that of the raw material gas. The zone in the reactionfurnace is heated to 700 to 1,300° C., preferably 1,000 to 1,300° C.

Usually, a cylindrical electric furnace is used as the reaction furnace.The furnace preferably includes, for example, pipes or tubes throughwhich the raw material gas and the carrier gas are fed, such that bothgasses are fed directly to a zone heated at a predetermined temperature.In this case, a pipe for feeding the raw material gas and a pipe forfeeding the heated carrier gas are preferably separated from each other,in order to facilitate control of the temperature of the raw materialgas. As used herein, the phrase “the raw material gas is instantaneouslyintroduced” refers to the raw material gas being fed to the reactionfurnace through a pipe such that the time during which the gas is heatedin the pipe at or above the decomposition temperature of the organictransition metal compound is 0.5 second or less, preferably 0.1 secondor less. In order to control the temperature of the raw material gas asdescribed above, the pipes for feeding both gasses must be provided inthe zone heated at a predetermined temperature such that the ends of thepipes are close to each other, and if necessary, the pipes are thermallyinsulated.

When the raw material gas introduced into the reaction furnace thermallydecomposes, the organic compound serves as a carbon source, andtransition metal particles derived from the organic transition metalcompound serve as a catalyst, to thereby produce fine carbon fiber onthe transition metal particles serving as a nucleus. If necessary, theresultant carbon fiber is optionally subjected to heat treatment at 900to 3,000° C., more particularly at 900 to 1,900° C. or 2,000 to 3,000°C. depending on the utilities, to thereby obtain a unique fine carbonfiber of the present invention.

The heat treatment may be carried out once or in plural times stepwise.The maximum temperature and retention time of the heat treatmentdetermine the physical properties, structure and the like of theresulting carbon fiber. The retention time is influenced by the type ofapparatus, the treating amount, density, diameter and aspect ratio ofcarbon fiber and the like and thus is not generally determined. However,usually the heat treatment is carried out for several minutes to severalhours, preferably 10 minutes to about 3 hours.

The heat treatment may be carried out in a customary electric furnace.In order to prevent nitridation of the surface of the carbon fiber, theheat treatment is preferably carried out in an inert gas atmosphereother than N₂ (for example, argon, helium or the like). The heattreatment renders the carbon fiber to have a more excellent conductivitythan the fiber filaments obtained without heat treatment.

Fine carbon fiber of the present invention will now be described.

The carbon fiber of the present invention is featured in:

-   1) a fine carbon fiber having an outer diameter of about 1 to about    80 nm and an aspect ratio of 10 to 30,000, comprising a hollow    center shaft and a multi-layer sheath structure of a plurality of    carbon layers, the layers forming annual rings, wherein the    sheath-forming carbon layers form an incomplete sheath, i.e., the    carbon layers are partially broken or disrupted in a longitudinal    direction, and the outer diameter of the fiber and/or the diameter    of the hollow center portion are not uniform in a longitudinal    direction;-   2) a fine carbon fiber according to 1) above, wherein the thickness    or structure of the carbon layers is partially asymmetrical with    respect to the hollow center portion; and-   3) a fine carbon fiber having an outer diameter of about 1 to about    80 nm and an aspect ratio of 10 to 30,000 comprising the fine carbon    fiber as recited in 1) or 2) above in an amount of about 10 mass %    or more.

The fine carbon fiber has a structure similar to that of carbon fiberproduced through the aforementioned conventional vapor-growth process,but the fiber of the present invention is characterized by the followingpoints.

Firstly, with regard to the structure, the carbon fiber has amulti-layer sheath structure of a plurality of carbon layers formed fromcarbon atoms, in which the layers form like annual rings. The carbonlayers are formed of regularly ordered carbon atoms. FIGS. 1A and 1Bshow schematic representations of the carbon fiber as observed under atransmission electron microscope (TEM) in a longitudinal direction. Asshown in these figures, most carbon layers are grown in a horizontaldirection (longitudinal direction) so as to form a plurality of directlines. Therefore, the structure of the carbon fiber seems to be similarto that of a conventional vapor-grown carbon fiber. However, the carbonfiber of the present invention differs from the conventional carbonfiber in that the sheath-forming carbon layers are partially broken ordisrupted in a longitudinal direction. FIG. 3 illustrates a conventionalvapor-grown carbon fiber including a hollow center portion 11, andcarbon layers 12 forming concentric rings, which layers are regularlygrown symmetrically with respect to the center shaft A–A′. FIG. 4illustrates the fine carbon fiber including carbon layers 12, whichlayers are asymmetrical with respect to a center shaft A–A′. As shown inFIG. 4, the carbon layers form an incomplete sheath, and most carbonlayers are partially lost or disrupted between adjacent sheath-formingcarbon layers.

The carbon fiber of the present invention is similar to the conventionalcarbon fiber in that the fiber contains a hollow center portion 11.However, the carbon fiber of the present invention is characterized inthat the inner diameter d₂ of the center portion 11 is not uniform.

In the fine carbon fiber of the present invention, the carbon layers 12are asymmetrical with respect to the center axis A–A′ of the hollow part11 in the central part thereof; the thicknesses e of the layers 12differ from portion to portion. For example, the carbon layers 12 arethickened so as to increase the outer diameter of the carbon fiber or toreduce the inner diameter of the hollow center portion 11 as comparedwith an imaginary complete cylindrical sheath, resulting in that theouter diameter d₁ of the carbon fiber or the diameter d₂ of the hollowcenter portion 11 varies greatly in accordance with variation in thethickness e. Variation in the outer diameter d₁ or the diameter d₂ mayreach about 10 and some % of the minimum diameter of the filament insome portions in which the thickness e varies greatly and about 2 to 3%of the minimum diameter of the carbon fiber in some portions in whichthe thickness e varies slightly. These portions where variations inthickness e are observed form protrusions extending in some portions ina longitudinal direction of the carbon fiber. As described above, acharacteristic feature of the fine carbon fiber resides in that thecarbon fiber does not have a perfect columnar shape.

In a portion of the carbon layers in which the thickness e is large, thecarbon layers 12 disrupted in a longitudinal direction entertherebetween, and the number of layers increases. Alternatively, in sucha portion, end surfaces of the carbon layers 12 are exposed to theoutside. As used herein, the term “the thickness e of the carbon layers12” refers to the distance between a position at the circumference ofthe carbon fiber and a position at the circumference of the hollowcenter portion 11, the distance being measured in a radial direction ofthe carbon fiber.

The fine carbon fiber of the present invention was embedded and fixed ina heat curing resin, and ground to form a cross-sectional surfaceperpendicular to the direction of carbon fiber (longitudinal direction).The cross-sectional surfaces were observed on transmission electronmicroscope. FIGS. 5A and 5B are transmission electron micrographs of thecross-sections. As will be apparent from FIGS. 5A and 5B, the shape ofcross-section is polygonal and annual rings of sheath of carbon layers(carbon sheets) arranged concentrically around the hollow part are boundto each other.

When the hollow center portion 11 at a portion in which the thickness eof the carbon layers 12 varies is subjected to electron diffraction, theresultant diffraction profile is asymmetrical. At the portion, thestructures of the carbon layers differ partially; i.e., the structure ofthe carbon fiber is not uniform.

Measurement of Raman spectrum of the fine carbon fiber of the presentinvention revealed that the intensity ratio R of so-called D peak havingan absorption in the vicinity of 1,360 cm⁻¹ to so-called G peak havingan absorption in the vicinity of 1,580 cm⁻¹ (I_(D)/I_(G)) is about 0.6to about 1.6 for the carbon fiber subjected to the heat treatment at 900to 1,500° C. and about 0.1 to about 1 for the carbon fiber subjected to2,000 to 3,000° C.

The value R could not reach 0.1 or less no matter how high thetemperature of the heat treatment was.

Furthermore, the interplane distance C₀ by X-ray diffraction accordingto the Gakushin method (Carbon, No. 36, p.25–34 (1963)) was 6.70 to 6.95Angstroms (0.670 to 0.695 nm) for the fiber subjected to the heattreatment at 900 to 1500° C. and 6.70 to 6.90 Angstroms (0.670 to 0.690nm) for the fiber subjected to the high temperature heat treatment at2,000 to 3,000° C.

As described above, the fine carbon fiber of the present invention has anon-uniform outer diameter or an imperfect columnar shape. Therefore,when the carbon fiber is incorporated into resins, rubbers or the like,the carbon fiber exhibits an excellent adhesion to the resin or thelike, as compared with conventional carbon fibers, so that the carbonfiber can be added to the resin as a filler material without the carbonfiber being subjected to any pre-treatment.

When the fine carbon fiber of the present invention is incorporated, asa conducting filler, into resins in an amount of about 10 mass % ormore, preferably about 15 mass % or more, adhesion of the carbon fiberto the resin, rubber or the like is improved due to the structuralfeature of the carbon fiber.

The fine carbon fiber of the present invention is obtained as anelongate fiber having an outer diameter of about 1 to about 80 nm and anaspect ratio of 10 to 30,000. Therefore, the carbon fiber can becompounded as a filler material in large amounts, and the carbon fiberexerts excellent reinforcing effects and is excellent in processability.

As described above, some end surfaces of the carbon layers are exposedto the outside (open). Therefore, when it is used as an additive in abattery, the carbon fiber of the present invention exhibits an excellentability to catch ions. Furthermore, the conductivity of the carbon fiberis the same as that of conventional vapor-grown carbon fibers. Inaddition, the carbon fiber of the present invention has a rough surface,and thus exhibits an excellent wettability to an electrolytic solutionin a battery. Therefore, the carbon fiber of the present invention issuitable as an additive to be incorporated in a battery.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described in more detail by way ofexample, which should not be construed as limiting the inventionthereto.

EXAMPLE 1

FIG. 2 shows a schematic representation of a production apparatus forproducing a fine carbon fiber. The apparatus includes a vertical heatingfurnace 1 (inner diameter: 170 mm, length: 1,500 mm); a raw materialfeed pipe 4 for introducing a raw material vaporized in a raw materialvaporization apparatus 5; and a carrier gas feed pipe 6 for feeding acarrier gas heated in a carrier gas heating apparatus 7, the pipes beingprovided on the top of the furnace. The raw material feed pipe 4 isprovided such that an end thereof is adjusted to be disposed at aposition in a temperature region in the furnace where the temperature is1,000° C.

A benzene solution containing ferrocene (4 mass %) and thiophene (2 mass%) was vaporized, and fed through the raw material feed pipe 4 at a rateof 18 g/minute to the heating furnace while the temperature of thevaporized solution was maintained at 200° C. Separately, hydrogenserving as a carrier gas was heated to 600° C. in the carrier gasheating apparatus 7, and fed at a rate of 100 L/minute to the heatingfurnace, and the both gases were reacted at 1,000° C.

The fine carbon fiber produced through the reaction was collected, andsubjected to heat treatment at 1,300° C. for 20 minutes in an Aratmosphere. Then a portion of the resultant carbon fibers was furthersubjected to heat treatment at 2,800° C. for 20 minutes in an Aratmosphere.

The both carbon fibers obtained through heat treatments at 1,300° C. and2,800° C., respectively, have a multi-layer sheath structure in whichlayers of carbon atoms are superposed one on another and the carbonlayers are partially broken or discontinuous in a longitudinal directionas observed by TEM and shown in FIGS. 5A and 5B. In addition, the carbonlayers have portions where the sheath-forming carbon layers constitutingthe multilayer structure differ in thickness at positions symmetricalwith respect to the center axis of the hollow center portion (left handand right hand positions in FIGS. 5A and 5B).

Most carbon fibers produced through the above production process had anouter diameter falling within a range of about 10 to 50 nm and an aspectratio of tens or more. The majority of the carbon fibers had astructural feature such that the sheath-forming layers of carbon atomswere disrupted and the outer diameter was not uniform. There were alsoobtained carbon fibers in which variations in the outer diameter of eachfiber and in the diameter of a hollow center portion of the fiber were10 and some percents (%).

The conductivity of the fine carbon fiber of the present invention wassimilar to that of conventional VGCF having a diameter of 100 nm ormore.

INDUSTRIAL APPLICABILITY

The fine carbon fiber according to the present invention differs fromconventional carbon fibers such as PAN or conventional vapor growncarbon fibers (VGCF), and have an outer diameter as small as about 1 toabout 80 nm and an aspect ratio of 10 to 30,000. Since sheath-formingcarbon layers constituting each carbon fiber are partially disrupted,the carbon fibers can be used as the conducting filler in resins,rubbers and the like without the carbon fibers being subjected tosurface treatment. Alternatively, since the carbon fiber of the presentinvention exhibits excellent wettability to an electrolytic solution,the carbon fiber can be used as an additive in a battery.

1. A fine carbon fiber having an outer diameter of about 1 to about 80nm, an aspect ratio of 10 to 30,000, R value (I_(D)/I_(G)) by Ramanspectrophotometry of about 0.6 to about 1.6, and an interplane distanceC₀ by X-ray diffraction of 6.70 to 6.95 Angstroms, and having across-section perpendicular to the longitudinal direction of the carbonfiber being of a polygonal shape, comprising a hollow center portion andsubstantially a multi-layer sheath structure of a plurality of carbonlayers.
 2. A fine carbon fiber having an outer diameter of about 1 toabout 80 nm, an aspect ratio of 10 to 30,000, R value (I_(D)/I_(G)) byRaman spectrophotometry of about 0.1 to about 1, and an interplanedistance C₀ by X-ray diffraction of 6.70 to 6.90 Angstroms, and having across-section perpendicular to the longitudinal direction of the carbonfiber being of a polygonal shape, comprising a hollow center portion andsubstantially a multi-layer sheath structure of a plurality of carbonlayers.
 3. The fine carbon fiber as claimed in claim 1 having across-section perpendicular to the longitudinal direction of the carbonfiber being of a polygonal shape, comprising a hollow center portion anda multi-layer sheath structure of a plurality of carbon layers in theform of annular rings around the hollow part.
 4. The fine carbon asclaimed in claim 2, having a cross-section perpendicular to thelongitudinal direction of the carbon fiber being of a polygonal shape,comprising a hollow center portion and a multi-layer sheath structure ofa plurality of carbon layers in the form of annular rings around thehollow part.
 5. Fine carbon fiber having an outer diameter of about 1 toabout 80 nm and an aspect ratio of 10 to 30,000, comprising the finecarbon fiber as claimed in any one of claims 1 to 4 in an amount ofabout 10 mass % or more.